by David Kirp

Heredity versus environment, nature versus nurture: the argument over what best explains intelligence has been going strong for more than a century. It's been a nasty fight, with each side questioning the other's bona fides, and it's not just an academic squabble. What's at stake isn't simply the definition of good science but the meaning of the just society.

Neuroscientists have been witness to how the architecture of the brain is transformed by experience, especially during the first years; that's why leaders in that field have been promoting early education. But geneticists have been the naysayers. A century's-worth of genetics research concludes that a person's IQ is remarkably stable from birth, and that as much as 80 percent of IQ differences can be attributed to heredity. As Sandra Scarr, former president of the Behavior Genetics Association, memorably put it, the world a child lives in matters only for those who are "trapped in crack houses in inner cities" or "locked in basements or attics by vengeful, crazy relatives." Otherwise, genes rule.

"The Bell Curve," which sold more than half a million copies when it was published a decade ago, popularized this "all in the genes" argument. If these researchers were right, then early education, far from being a great equalizer, was merely a waste of money. Small wonder, they argued, that Head Start programs showed meager results - it was attempting the impossible, and so led to a dead end.

Here's the scientific news, with potent policy implications: a new generation of studies demonstrates that nature and nurture don't occupy separate spheres. Much of what is labeled hereditary becomes meaningful only in the context of experience. Rather than nature versus nurture, as the debate has been framed, it's nature through nurture.

Evaluations of the decades-long impact of high quality pre-kindergarten programs - iconic studies of Perry Preschool, the Abecedarian project, Chicago's Child-Parent Centers and the like - show that such initiatives can make a lifelong difference. The new genetics, like the neuroscience research of the past generation, helps to explain why. It adds the imprimatur of science to the argument for early education. As "A Science-Based Framework for Early Childhood Policy," a just-released report from the Center on the Developing Child at Harvard University (www.developingchild.harvard.edu), concludes, "a remarkable convergence of new knowledge about the developing brain, the human genome, and the extent to which early childhood experiences influence later learning, behavior and health now offers policymakers an exceptional opportunity to change the life prospects of vulnerable young children.

The old-line geneticists suffered from a major blind spot. They mostly studied the experiences of middle-class children; not surprisingly, they discerned no social class effects. But when researchers started comparing the lives of twins from poor families with those of middle-class twins (twin studies are the gold standard in the field), they learned that whether a child's genetic potential is fully realized depends heavily on the circumstances in which he or she is raised. Nature may fix the boundaries of what a child may achieve - to that extent the old-line thinking is right - but nurture determines whether that potential will be realized.

An affluent 3-year-old child has a larger working vocabulary than the welfare mother of a 3-year-old - and by her fourth birthday, she'll have heard 30 million more spoken words. Even a middle-class youngster growing up in a fraught home is likely to benefit from a good nursery school and supportive adults outside the family, and that helps to blunt the detrimental impact of a difficult home life. The effects of social class are felt all the way up the social ladder - the better-off the family, the more cultural capital to which their children will be exposed. Genetics is essentially everything for children from well-off homes - they "max out" their potential, says psychologist Eric Turkheimer, who conducted this research - but for poor kids, it's the variation in their experiences that prove crucial.

So, too, for adopted children, who have been the focus of much genetics research - change their social worlds, as studies by French psychologists Christiane Capron and Michel Duyme have shown, and their IQ is likely to change as well. Rags to riches and richest to rags - the effect is essentially the same. One study found that poor children adopted by well-off parents had IQs comparable to those of children from wealthy parents adopted by poor families. In another study, 5-year-olds, who had been abused and abandoned early in their lives, were adopted by well-off families. A decade later, their average IQs increased from 76, at the cusp of retardation, to a whisker below normal - something undreamt of by conventional geneticists.

Nature through nurture: on this key point, the scientific disciplines are converging. Neuroscientists have used brain scans to find that while middle-class kids generally reach their full neuronal potential, for poor kids it's another story; differences in environment are critical. And molecular geneticists have shown that whether a genetic propensity - for aggression, say, or thrill-seeking - emerges will depend heavily on the world in which a child grows up. The form, or allele, of these genes actually varies with the environment: a child born with the genetic propensity to be aggressive, raised by punitive parents, is likely to be a troubled adult, but that propensity will be muted if such a youngster grows up in a nurturing home. There's also new evidence of the malleability of IQ. Over the past 30 years, the black-white IQ gap has narrowed; that's not supposed to happen, according to the classic genetics model. And several recent studies of large-scale state-run preschool programs confirm that high-quality early education has enduring effects on how kids fare in school.

This emerging scientific consensus makes hash of the Bell Curve argument that it's a waste of time to focus on the needs of poor kids. On the contrary, the research shows the possibility of having a profound impact on these children's lives by giving them a shot at the stimulating world that middle-class youngsters routinely inhabit. Don't all children deserve that chance?